SAN JUAN, Puerto Rico — Flush from two Southern victories, Rick Santorum held up his successes as proof that GOP front-runner Mitt Romney is a "fundamentally flawed" candidate. Romney chugged along raising and spending money for his effort to wrap up the nomination race as it spreads to far-flung contests in Puerto Rico, Illinois, Louisiana and beyond.

"If we keep winning races, eventually people are going to figure out that Gov. Romney is not going to be the nominee," Santorum told reporters gathered Wednesday outside the governor's mansion in San Juan.

His message was weakened somewhat by coming just after he paused for a private meeting inside the mansion with Gov. Luis Fortuno, a friend from their shared days in Washington who also happens to have endorsed Romney.

Santorum's wins in the Alabama and Mississippi primaries Tuesday night effectively shut the door on Newt Gingrich's strategy of resuscitating his campaign in the South, where he hoped to establish himself as regional favorite son.

Santorum would not say it was time for Gingrich to quit the race. But his campaign advisers have made plain their wish that the former U.S. House speaker from Georgia step aside and stop drawing the same conservative votes Santorum needs.

Santorum and Gingrich accounted for an overwhelming majority of votes in the two Southern primaries Tuesday, while Romney held his ground in the delegate chase by winning caucuses in Hawaii and American Samoa.

Santorum said the former Massachusetts governor should be winning easily, given his overwhelming financial advantage.

"There's something fundamentally right with what we're doing and the message we're delivering and there's something fundamentally flawed about the person we're running against," he said of Romney.

Unbowed by two third-place finishes, Romney noted his impressive lead in the delegate race after Tuesday's four contests. "I am pleased that we will be increasing our delegate count in a very substantial way after tonight," he said in a statement.

Earlier, Romney told CNN that Santorum was "at the desperate end of his campaign."

The race now turns to caucuses Saturday in Missouri, where Santorum and Romney already have invested substantial time, and Puerto Rico's primary on Sunday, which is drawing rare attention in a year when the race has drawn out longer than most.

Romney wasn't slated to arrive in the U.S. territory until Friday, after spending two days in New York City raising money.

Romney already is spending to dominate the airwaves in Illinois, which will offer a hefty 54 delegates in voting Tuesday. His campaign has put down almost $1 million for Illinois TV advertising, on top of $2.4 million spent there by a supportive super PAC.

Romney's seemingly unassailable delegate lead left his opponents' campaigns talking about less orthodox ways to stop him. John Brabender, senior strategist for the Santorum campaign, said many of the delegates weren't bound and could still switch their votes to Santorum.

"The fact is, in both states, the conservative candidates got nearly 70 percent of the vote," Gingrich crowed after placing second in Alabama and Mississippi. "If you're the front-runner and you keep coming in third, you're not much of a front-runner."

Romney had been hoping for at least one Southern victory Tuesday that might have allowed him to start arguing it was time for the party to gather behind him and begin the general election fight against President Barack Obama.

Instead, Romney now faces a resurgent Santorum — and he is without the overwhelming financial advantage he boasted throughout the early states. Romney's campaign trimmed some spending in recent weeks as he was forced to spend more time campaigning and less time fundraising. Still, he's got more delegates than his rivals combined.

Santorum's victories Tuesday were worth at least 35 delegates, but Romney won at least 41. Gingrich won at least 24, while Ron Paul picked up at least one. The delegate split underscored the difficulty Romney's rivals face in overcoming his big lead.

The partial allocation of delegates from Tuesday's voting states left Romney with 495 in The Associated Press count, out of the 1,144 needed to win the nomination. Santorum had 252, Gingrich 131 and Paul 48.

And while Santorum in particular challenges the mathematical projections, Romney is amassing delegates at a rate that puts him on track to clinch control of the nomination before the convention next summer.

A senior Romney adviser, Jim Talent, said the campaign is where it needs to be. "We're really running against the delegate totals more than any of the others," Talent told CNN on Wednesday.

Gingrich deputy campaign manager Vince Haley suggested the former House speaker was putting himself in position to compete at a brokered convention, saying Gingrich could "win a debate in this country over the course of the rest of this campaign."

It is rare for Alabama and Mississippi to play an important role in a Republican nominating campaign, but the 2012 race has gone on far longer than usual.